1. Field of the Invention
The present invention relates to an apparatus and method for the treatment of gas streams containing volatile organic compounds (VOCs) involving catalytic oxidation of the VOCs, in a point-of-use (POU) system, suitable for applications such as fabrication of semiconductor materials and devices, as well as products such as flat panel displays or other microelectronics products, manufacturing of compact discs (CDs) and other storage and memory devices, and photolithography applications involving oligonucleotide characterization.
2. Description of the Related Art
In the field of treatment of gaseous effluents in the manufacturing of semiconductor materials, devices, and products, storage and memory articles, and the use of photolithography for oligonucleotide characterization, a wide variety of effluent gases are produced in the process facility.
A large number of these effluent gases contain VOCs, such as alkanols, organics, photoresists, and breakdown products of photoresists and other reagents, and a wide variety of other gases which are desirably removed from the waste gas streams produced in the process facility, before the waste gas is vented from the process facility to the atmosphere.
The normal options for the treatment of VOC-containing gas streams include combustion and catalytic oxidation, which in either case may further include preliminary concentration of the gas stream VOC components to be removed by the treatment process.
Combustion processes are old and well-established in the art for the treatment of VOC-containing waste gas streams, however, catalytic oxidation processes are in increasingly widespread use as a result of their high efficiency and cost-effectiveness. By catalytically converting the VOCs oxidatively to the combustion products carbon dioxide and water, the catalytic oxidation process affords an effective solution to the problem of VOC-containing waste stream treatment.
One problem facing catalytic oxidation technology in applications such as those described hereinabove is the wide fluctuation in the composition of the waste gas stream, as various unit operations are carried out in the process plant. For example, in semiconductor manufacturing operations, the manufacturing facility may include workstations comprising tools for sequential unit operations on wafers or other substrate structures. The workstation may carry out a given sequence of operations using specific solvents, reagents, etchants, dopants, masks, photoresists, etc. in the various constituent steps of the manufacturing process.
As a result, the composition of the waste gases generated at a given workstation may vary widely over time, as the successive process steps are carried out, and the effluents generated in the workstations in the manufacturing facility may vary widely at any given time, among the various workstations and constituent sectors of the manufacturing facility.
Faced with this variation of the composition of the waste gas streams from the process facility, and the need to adequately treat the waste gas on a continuous and ongoing basis during the operation of the facility, it has been a common approach to provide a single, overdesigned (in terms of treatment capacity), large-scale waste gas treatment system for an entire process facility, which can continuously treat the waste gas in the form of a combined stream yielded by consolidation of all the individual workstation waste gas streams in the facility. Such large-scale catalytic oxidation units, receiving all the waste gas from the entire process facility, typically are disposed in proximity to final venting means such as vent gas stacks, for efflux of the final treated VOC-depleted effluent to the atmosphere. Roof-mounted catalytic oxidation systems are commonly employed for such purpose.
By such large-scale treatment of a single consolidated gas stream deriving from the entire process facility, the oscillations and peaks of specific components of waste gas species generated in the facility are to some extent damped. In consequence, the loading of VOC from particular waste gas species on the oxidation catalyst in the treatment system is maintained at a lower peak value, due to dilution or spreading effects.
Considering the thermal and thermodynamic aspects of operating such single unit, large-scale VOC treatment systems, the waste gas stream from the process plant in order to carry out efficient catalytic oxidation of the VOC content of the waste gas stream, must be heated to an appropriate elevated temperature for catalytic oxidation. Additionally, the catalytic oxidation process is strongly exothermic in character, so that substantial heat is generated in the operation of the catalytic oxidation process. Accordingly, economic operation of the catalytic oxidation process requires that a substantial portion of the heat generated by the catalytic oxidation be recovered and re-used within the process, to bring the influent gas stream to an appropriate level for catalytic oxidation of the VOC therein while concurrently reducing the temperature of the effluent from the catalytic oxidation process to an appropriate level for ultimate discharge of the finally treated gas to the atmosphere.
The efficient operation of the catalytic oxidation process system also requires the achievement and maintenance of appropriate operating temperature in the bed of oxidation catalyst during catalytic oxidation of the VOC-containing gas.
As a result of the highly exothermic character catalytic oxidation, oxidation catalyst must be provided in sufficient quantity to thermally accommodate the overall heat balance in the process system, irrespective of fluctuations in the composition of the effluent gas stream being treated. Further, dilution air addition to process streams in the plant facility significantly increases the volume and volumetric flow rate of the effluent which is treated to remove VOCs therefrom. Such dilution air may derive from air added to the VOC-containing effluent upstream of the catalytic oxidation unit to provide control the temperature in the exothermic oxidation reaction, and prevent thermal "runaway" conditions from occurring. The dilution air content of the effluent gas stream may also derive from introduction of air as a carrier or "sweep" gas in specific processing operations in the plant facility, by which VOCs and other components are entrained in the air stream for subsequent effluent treatment. Accordingly, the catalytic oxidation beds in large scale, single unit catalytic oxidation systems are typically greatly oversized, relative to the size and scale of oxidation catalyst beds which would otherwise be minimally required for treatment of an effluent stream from the process plant as a whole, at average operating conditions, average concentration levels, average composition of VOCs in the effluent gas stream, and without large surplusage of dilution air in the effluent gas being treated.
If such large scale, single unit catalytic oxidation system is not oversized, then any substantial fluctuation increases in the amount of catalytically oxidizable matter in the waste gas stream may result in undesirable overheating in the catalytic oxidation bed in the treatment system, as well as the potential development of thermal runaway conditions.
With the provision of large-size overdesigned beds of oxidation catalyst to accommodate wide variations in the concentration and types of VOC loadings in the effluent gas stream, it is necessary to concurrently provide make-up heat in the catalytic oxidation system upstream of the catalytic oxidation bed, so that the temperature of the gas stream being introduced to the oxidation catalyst is at a desired elevated temperature level even when the gas stream has a relatively low concentration of VOCs. Consequently, it is conventional practice to continually add in natural gas, propane, or other fuel to the effluent gas stream, and to provide supplemental heating, to maintain set point operating conditions as nearly as possible in the catalytic oxidation operation. In various catalytic oxidation systems, it is also common to add diluent gases to the effluent gas stream, to control temperature in the catalytic oxidation process. In the operation of conventional catalytic oxidation systems, it is desired to recover sufficient heat from the catalytically oxidized gas stream discharged from the catalytic oxidation bed, so as to achieve superior process efficiency, minimize the amount of added heat, and maintain a good overall thermal energy balance in the process system.
Due to the size and overdesigned character of the catalytic oxidation systems of the prior art, as required to treat cyclical loads of various VOC species, the efficiency of such systems in normal day-to-day operation is quite low. Further, when such effluent treatment systems encounter a gas stream component which poisons the oxidation catalyst, e.g., hexamethyidisilazane, the process plant's effluent treatment system is compromised, and the replacement of the extremely large inventory of catalyst is time-consuming, costly, and labor-intensive. Such disadvantages also inhere in the normal change-out of oxidation catalyst, since if an entire process facility is dependent on a single catalytic oxidation system for effluent treatment, the shut-down of the catalytic oxidation system for any reason means that the entire process facility must be taken off-line, or else the waste gases requiring treatment must be stored until the effluent gas treatment capability is restored.
In sum, the typical unitary VOC abatement systems for treatment of consolidated VOC-containing streams for an entire process facility are large units which treat relatively low ppmv concentrations of organic pollutants. These systems treat large volumes of air (30,000 cfm and more). As a result, they are expensive to purchase, expensive to install and costly to operate. Such unitary large-scale systems typically use flame oxidation to convert the VOC to CO.sub.2 and water, which can often lead to undesirable by-products such as NO.sub.x.
In addition, there exist a number of industries, e.g., the semiconductor industry, in which gas streams are treated to remove VOC components which originate from specific "point sources," i.e., single tools, individual processing operations, etc., within the plant facility, and by addition of such point source effluent gases to other flows of gases in the facility, a combined or consolidated effluent stream is yielded which then is subjected to final treatment for VOC removal. Thus, when a VOC fume is traced back to the originating point source, it may be found that significant quantities of dilution gases and other, e.g., non-VOC-containing, fume streams have been added to the original point source VOC-containing gas, to constitute an overall effluent gas flow stream for treatment.
In the semiconductor industry, VOC fumes may derive from photolithography tracks, isopropyl alcohol (IPA) dryers, organic spray resist strip tools, wet bench-based resist strip tools, spin-on-glass (SOG) tracks, various coaters, and parts cleaning wet benches of the semiconductor manufacturing facility ("fab"). Typically, these tools are batch process tools, and they may be chambered in order to allow sequential processing of wafers through the tool.
It is also typically found that only a relatively small number of the chambers of each tool actually contain VOCs. The net result is that a large end-of-line flow contaminated with trace quantities of VOC may be traceable back to a few point sources of VOCs which generate a high fraction (e.g., 97%+) of the VOCs being emitted from the fab. Significantly, these point sources can be readily determined and predicted through knowledge of the physical operation of the tool and through emissions characterization (using gas chromatograph, flame ionization detector (FID), etc.) of the gas flows coming from the tool.
Most importantly, these point source emissions are contained within low flow rates of air which are substantially less than the total flow rate of effluent (fumes and air) being emitted at the end of the effluent gas flow line. The total end-of-line gas flow thus may comprise a combined gas stream, whose constituent gas components derive from sources such as (i) the admission of dilution air (such as cabinet sweep air) into solvent duct headers in the process facility, (ii) fumes produced in processing equipment in which reagents, reactants, products, and/or other chemical components have very low vapor pressures, (iii) fumes produced by tools which are rarely operated, and (iv) fumes produced by tools with which solvents have been used that are no longer classified as ozone precursors by governing laws and/or regulations, and thus may be treated in combination with other gas species.
By treating the emissions directly from the aforementioned point sources in a point-of-use (POU) fashion, as hereinafter more fully described, the majority of the mass of VOCs being emitted from the processing facility can be destroyed, but with the processing of only a small fraction of the flow which otherwise would require treatment if the entire end-of-line flow from the processing facility were treated. The total VOC abatement fume flow processed by utilizing a POU approach versus an end-of-line approach may be as great as 10 to 1, or even greater.
It would therefore be a significant advance in the art, and accordingly is an object of the present invention, to provide an apparatus and method for the catalytic oxidation treatment of gas streams containing VOCs, in a point-of-use system which is constructed and arranged to treat only the effluent from a single workstation unit or tool in the process facility, so that the system is readily and simply adjustable to accommodate the specific waste gas effluent from a given workstation, tool, unit operation, or sequence of operations, within the process facility.
Such a point-of-use system would have the following advantages relative to large scale, single unit catalytic oxidation systems of the prior art: (1) the volume of the effluent gas stream being treated would be substantially reduced; (2) the oxidation catalyst required would be relatively small in volume and could be readily, simply, and economically replaced, without shut-down of the entire plant's effluent treatment system; (3) the process conditions for effluent gas treatment could be more simply, reliably and economically controlled, to achieve high efficiency effluent gas treatment, (4) the effluent gas treatment system could be more efficiently designed, and significantly more compact; (5) diluent gas requirements for temperature control of the catalytic oxidation operation could be significantly reduced; (6) since a point-of-use catalytic oxidation system would be significantly smaller in size and more efficiently designed and operated, such system would be significantly more cost-effective than a large-scale system for an entire process plant; and (7) such cost-efficiency of the point-of-use system would favor flexibility in expansion and modification of the process plant, since otherwise the large-scale catalytic oxidation system for an entire process plant must be overdesigned not only operationally to accommodate gross fluctuations in the composition, temperature and flow rate of the stream being treated, but it also must be overdesigned if it is to accommodate future expansion of the process plant, without wholesale revamping of the effluent treatment system (by contrast, process plant expansion with point-of-use systems would merely entail addition of discrete point-of-use gas treatment modules).
It would therefore be a significant advance in the art, and is an object of the present invention, to provide a catalytic oxidation system that is of a point-of-use character.
It is another object of the present invention to provide such a point-of-use catalytic oxidation system that is autothermal in character.
As used herein, "autothermal catalytic oxidation" means catalytic oxidation of a VOC-containing effluent gas stream, yielding a hot treated gas stream of reduced VOC content from which heat is recovered and used for heating the VOC-containing effluent gas stream to an appropriate elevated temperature for catalytic oxidation, without the addition of externally supplied heat. In other words, under autothermal catalytic oxidation conditions, no heating of the VOC-containing effluent gas stream is required beyond the heating of such gas stream with the heat recovered from the catalytically oxidized gas stream.
Autothermal catalytic oxidation of VOC-containing gas streams has not been successfully practiced in the prior art because the large scale, single unit catalytic oxidation systems of the prior art must be operated well below the autothermal threshold. Specifically, sub-autothermal operation is required to avoid generation of excess heat and the risk of thermal runaway under widely fluctuating VOC concentration and loading conditions. Accordingly, significant supplemental heating of the influent VOC-containing gas stream is necessary to maintain the catalytic oxidation bed at a sufficiently high temperature for efficient operation.
One of the principal difficulties in achieving practical autothermal operation is due to batch-wise variability in concentration of some fume streams. In some industries, such as the semiconductor industry, the VOC emitting tools may be operated to carry out a number of steps in a sequence; thus, in the fabrication of semiconductor devices, a wafer passing through a tool may be subjected to a multiplicity of process steps. These steps occur as a single wafer is processed in a chamber of the particular processing tool. By way of example, for a wafer undergoing coating with a photoresist, these steps may comprise: a) receipt of a new wafer on a coater bowl spin chuck, b) spinning of the wafer on the spin chuck, c) application of photoresist on the spinning wafer from a dispense arm, d) spinning excess photoresist off of the wafer, e) application of edge bead remover (top side and bottom side) to the wafer edge using a dispense arm, f) spinning off of the edge bead remover, g) application of a coater bowl rinse to clean residual photoresist and edge bead remover from the spinning wafer containment cup, and h) termination of spinning and removal of the wafer.
Because chemicals such as photoresist and edge bead remover typically have very high concentrations of volatile chemical solvents used as a carrier and coating uniformity enhancer for the non-volatile coater materials, pigments and photo-active ingredients which are desired to be coated on the wafer, simple vapor-liquid equilibria with the uniform air sweep passing over the wafer dictate that the evaporating solvent concentration will vary widely and in a transient fashion as the wafer is processed through each of the aforementioned steps.
During steps a) and b), very little solvent vapors will be emitted. During steps c) and d), very high levels of solvent will be emitted, as the material is applied and spun off. During steps e) and g), high levels of solvent will be emitted, but they will tend to be different chemicals than those emitted in steps c) and d). During step g), a high level of solvent of a different chemical may be emitted. During step h) emissions will be low since most of the solvent will have already evaporated.
In other segments of process facilities, different batchwise tools may be utilized. For example, in the specific case of an IPA dryer, the equipment system may consist of a bath of boiling IPA, above which is located a set of condensing coils, with a lid over the bath to prevent IPA vapor escape, a tray within which to load wafers to be dried, and an exhaust system to draw a constant flow of sweep air through the lid of the IPA dryer and over the top of the IPA bath, typically above the level of the condensing coils.
In such system, a typical wafer processing sequence may comprise: a) loading wafers into the tray with the lid closed, b) opening the lid, c) lowering the wafers past the condensing coils and into the IPA vapor area directly over the boiling IPA bath but below the level of the condensing coils, d) allowing the wafers to dry while in that IPA vapor region, e) retracting the tray of dried wafers up out of the IPA vapor zone and past the condensing coils, g) aspirating any liquid IPA which may have accumulated in the tray, h) opening the lid on the dryer, and l) fully retracting the tray of now-dried wafers to their original position.
Testing of process gas effluents has shown very wide fluctuations in concentration of the emissions coming from IPA dryers due to wide ranges of vapor-liquid equilibria conditions for each step of the above-described sequence. In one case it was found that emissions when IPA was aspirated from the tray were almost 5 to 10 times greater than the average concentration emitted during the overall wafer drying process cycle.
It is also important to note that such concentration variability within the process sequence cycle of each tool in the process facility is also exacerbated by a concentration variability between the various tool types which may be located at such facility. To choose a semiconductor fab as an typical example, emissions will be very different among IPA dryers, photolithography tracks, spin-on-glass coaters, organic spray resist strip tools, organic strip wet benches, and parts cleaning benches.
In addition, the chemistries may vary from track to track. In some applications, such as semiconductor fabs, chemistries may change widely and frequently, due to the constant influx of new mixtures offering advantages in processing capability and overall wafer processing reliability and lack of defects.
For example, one track may be processing a photoresist A with edge bead remover (EBR) 1 while a second track is processing an anti-reflective coating (ARC) B and an ARC EBR 2. The chemistries may be totally different and the emissions characteristics, which are dictated by the vapor-liquid equilibrium characteristics of the chemistries employed, may also vary considerably.
This process variability is further exacerbated by the fact that emissions may vary greatly from one manufacturer's tool to another manufacturer's tool. This may be due for example to subtle variations in interactions between air sweep rates over surfaces being coated and the chemicals being applied to the surface, air flow patterns over the surface of the wafer, different chemical application and dispense means and durations, different application and spin cycle times, different spin rates, etc.
The net effect of such variability is that it is very difficult to apply a single point-of-use VOC abatement technology to cover all the point sources within a semiconductor fab environment, or other processing complex, even though the advantages of pursuing a point-of-use approach are overwhelmingly great as compared to pursuing an end-of-line approach.
Because of the variety of tools in each facility and the wide variety of chemistries used in each tool, point-of-use VOC abatement equipment has not come into common usage, and instead VOC treatment has been carried out on bulk, dilute gas streams combined from constituent flows of VOC-containing gases from the entire plant facility, or with highly customized VOC treatment systems which by virtue of their customized character are of limited use in treating other gas streams or being shifted to other tools in a flexible manufacturing environment.
Highly customized VOC abatement systems are expensive, and do not enjoy the economics and ready availabilities/short lead times of mass production abatement systems. If a multiplicity of such customized VOC abatement systems are utilized, with each unit being deployed for point-of-use treatment of one of the numerous gas streams generated in the manufacturing or process facility, the attendant capital and operating costs of such customized approach can destroy the economic advantages which are conceptually afforded by point-of-use gas stream treatment.
Another factor which negatively impacts the economics of point-of-use VOC abatement is the effect of widely varying VOC concentration and its impact on the thermal management of the catalytic oxidation process. Because of high peak loads of VOC which typically are present in the operation of semiconductor fabs and other facilities generating VOC-containing gases, it generally is necessary to design the VOC treatment system equipment to accommodate the peak load (maximum VOC) condition. This is due to the fact that designing the system for a lower VOC load specification will result in over-temperature conditions developing in the system during VOC concentration excursions to high load levels.
Such over-temperature effects become especially significant when catalytic oxidation technology is employed, which requires that a minimum "light-off" temperature be maintained at the inlet to the catalyst. The net result is that such sudden and increasing VOC concentration excursions will generate a "pile-up" of enthalpy (translated to heat energy which is sensed as temperature) on the baseline light-off temperature.
Such sudden enthalpic increases and resulting over-temperature conditions make it particularly difficult to manage large-scale concentration excursions in a point-of-use system without designing to the maximum concentration and VOC load expected.
While such "maximum excursion" design practice is highly prudent to mitigate and minimize the effects of the over-temperature conditions developing during concentration excursions, it poses a significant dilemma for the designer in maintaining operating costs within economic limits. This is due to the fact that in "maximum excursion" design, the VOC oxidation system is based (sized, constructed, and operated) on the assumption that the VOC oxidation system can only be autothermal during maximum VOC concentration conditions.
During the remainder of the operating cycle, the VOC concentration will, by definition, be inadequate to support an autothermal condition. The net result will be that supplemental energy, in the form of electric heat or burner heat, will be required to make up for the enthalpy deficit present in the VOC catalytic oxidation unit during those periods of below-maximum VOC concentration. This sub-autothermal operation thus results in higher operating costs for the system, in terms of energy requirements for continuous operation.
In some environments, the utilities which are available for a point-of-use system may be more costly on a per unit basis than utilities which are available for an end-of-line system. This is because a point-of-use system is designed to be located in close physical proximity to the tool set, which may in turn be located in extremely clean, classified, or restrictive environments, e.g., within a semiconductor fab.
The end-of-line system typically is so massive that it could not possibly be located within these restricted environments, and therefore it is located outside where, advantageously, inexpensive utilities may be readily available for the system, without violation of building codes.
By way of example, the utility costs of electricity and hydrogen, commonly the only fuels available in a semiconductor fab, are on the order of $0.05 to $0.10 per kW-hr, and $82/MM BTU, respectively, while natural gas, which is readily available outside the semiconductor fab, may have a utility cost of $5 to $3/MM BTU.
The application of "maximum excursion" design, in which the VOC catalytic oxidation system is sized based on the highest possible effluent stream concentration of VOCs, to end-of-line VOC abatement systems has no significant detrimental impact on the economics of the end-of-line system, because it already is grossly overdesigned.
On the other hand, the application of such "maximum excursion" design principles has a tremendous impact on a point-of-use system and can wholly eliminate the overwhelming economic advantage otherwise offered in the deployment of point-of-use systems by virtue of their processing of greatly reduced volumetric gas flow rates (relative to end-of-line systems).
Accordingly, in a point-of-use system, it is of great advantage to operate the VOC oxidation system at a VOC inlet concentration which is closer to a time-averaged level for the stream than is even remotely approachable in end-of-line VOC abatement systems.
Relative to the apparatus and method of the present invention as hereinafter more fully disclosed, relevant art in the field of the invention includes the following: U.S. Pat. No. 5,326,631 to Carswell et al. (metal fiber/ceramic porous fiber burner); U.S. Pat. No. 5,439,372 to Duret et al. (surface combustion radiant burner with blue flame combustion zones surrounded by surface radiant combustion zones); U.S. Pat. No. 5,410,989 to Kendall et al. (watertube boiler system employing fiber matrix radiant burners which radiantly heat tube coils containing water); U.S. Pat. No. 5,211,552 to Krill et al. (flameless combustion of gaseous fuel with air in an amount of 50% to 150% in excess of the stoichiometric requirement, with the fuel/air mixtures being passed through a porous surface combustor to an adiabatic zone); U.S. Pat. No. 4,412,523 to Schreiber et al. (gas-fired, forced air furnace system including a heat exchanger in the air stream and a combustion chamber containing a fiber matrix element, and means for igniting gas-air mixtures to produce radiant heating and convection heating, with heat exchanger surface area preselected corresponding to excess air and heat input, to condense moisture in combustion gases, for achievement of at least ninety percent thermal system efficiency); and U.S. Pat. No. 4,481,154 to Gough et al. (multi-loop tube inserts for enhancing heat exchange efficiency).
Because of the wide variations in VOC loading from the different tools in the normal operation of the process plant in which VOCs are generated as waste gas components, as well as the wide variations within even tools of similar type depending on the manufacturer and chemistries used, and the inevitability of frequent changes in manufacturing chemistry in process facilities such as semiconductor fabs, as necessary to keep up with changing technology, it becomes extremely difficult to design a single "multiple-off" mass produceable system which will accommodate all tool emissions in an economic package.
The alternative is to introduce a high level of custom engineering to each unit, which places a tremendous cost burden on a fab which purchases a multiplicity of point-of-use VOC abatement units for incorporation with the various tools within the fab. This custom engineering cost disadvantage is exacerbated by the fact that point-of-use VOC abatement systems must be deployed as a constellation of numerous units scattered throughout the plant facility, rather than a single unit for such entire facility.
Such point-of-use VOC system cost disadvantage is further exacerbated by the frequently changing chemistries and tool sets within the fab environment. A unit which was designed for an autothermal condition on one tool with a given chemistry set, may be totally inadequate because the chemistries used in that tool changed, or because the tool was replaced with a different tool or put into a different service. These practices are very common in industries such as the semiconductor industry, and can necessitate substantial heavy retrofitting and high rework costs in order to maintain satisfactorily high VOC abatement efficiency of the system. If this retrofitting and rework is not performed, the catalytic oxidation VOC abatement system will either constantly "overthermalize" (experience over-temperature conditions) or with constant utility draw substantially deviate from optimal "autothermal" catalytic oxidation conditions.
In consequence of the aforementioned problems, the provision of a flexible point-of-use catalytic oxidation VOC abatement system, which can handle a wide variety of VOC duties very efficiently, and which possesses a design eliminating or substantially minimizing the necessity of custom engineering, would be a fundamental advance in the art of VOC abatement, and of utmost importance to the evolution and further progress of industries such as semiconductor manufacturing. Such a system would possess the ability to flexibly operate in a thermally efficient manner with widely varying tool sets, VOC concentrations, VOC-containing gas stream flow rates, and chemistries.
It is a further object of the invention to provide a flexible point-of-use catalytic oxidation VOC abatement system of such character.
It is yet another object of the invention to provide a point-of-use catalytic combustion system for the treatment of VOC-containing gases of varying type, wherein the system is embodied in an apparatus of a highly compact character, to accommodate the placement thereof in close physical proximity to the process equipment which is the source of the VOC-containing gas to be treated.
In one of the applications of VOC abatement systems, the product is supplied to the semiconductor industry on photolithography tracks. The process units which are employed, are designed to prepare a silicon wafer to receive a photomask coat, apply a thin photosensitive photomask to the silicon wafer, bake the mask which has been applied to the wafer, shuttle that wafer to a stepper which applies a pattern to the wafer through a masking and exposing process, receive the wafer back, wash the wafer with a developing solution which removes exposed photomask and leaves behind unexposed photomask (in the case of positive photoresists), or alternatively removes unexposed photomask and leaves behind exposed photomask (in the case of negative resist).
In some cases, special materials must be applied to prepare the silicon wafer surface for receiving the application of the photomask coating. In the case of silicon wafers, this surface priming material is typically hexamethyldisilazane (HMDS). The purpose of this priming material is to allow the photomask to be applied to the wafer and promote good adhesion. In this case, it is logical that this priming material be an organosilicate such as HMDS, since the objective is to promote the adhesion of an organic photomask to a silicon-based silicon wafer.
In some cases, materials other than HMDS may be used for priming the surface, but these also tend to be organosilicates of different forms and character, due to the fundamental requirement to bond an organic photomask to an inorganic substrate.
Such use of HMDS, when it occurs, has certain negative implications for VOC abatement practices which must be used to abate the fumes deriving from the associated process equipment. In the case of catalytic oxidation of VOCs, the HMDS will tend to mask the active surfaces of the oxidation catalyst and cause an eventual loss of performance. In the case of thermal oxidation, the HMDS may oxidize to form SiO.sub.2 particulates, which then act deleteriously to occlude heat exchangers and ducting.
Accordingly, it is a further object of the present invention to provide a VOC abatement system, in which HMDS is selectively removed and prevented from deactivating the oxidation catalyst and/or forming occlusive particulates which can damage the process system or lower its performance to unsatisfactory levels.
Additional objects and aspects of the present invention will be more fully apparent from the ensuing disclosure and appended claims.